Method of operating a charged particle beam specimen inspection system

ABSTRACT

A charged particle beam specimen inspection system is described. The system includes an emitter for emitting at least one charged particle beam, a specimen support table configured for supporting the specimen, an objective lens for focusing the at least one charged particle beam, a charge control electrode provided between the objective lens and the specimen support table, wherein the charge control electrode has at least one aperture opening for the at least one charged particle beam, and a flood gun configured to emit further charged particles for charging of the specimen, wherein the charge control electrode has a flood gun aperture opening.

CROSS-REFERENCE TO RELATED APPLICATION

This application is division of Ser. No. 14/446,146, filed Jul. 29,2014, the entire contents of which are incorporated herein by referencein its entirety for all purposes.

TECHNICAL FIELD OF THE INVENTION

Embodiments of the present invention relate to devices for imaging aspecimen, e.g. a wafer, with one or more charged particle beams andincluding a flood gun. Embodiments of the present invention particularlyrelate to a charged particle beam specimen inspection system having anobjective lens and a flood gun, specifically to a charged particle beamspecimen inspection system, a multi-beam specimen inspection system, anda method of operating a charged particle beam specimen inspectionsystem.

BACKGROUND OF THE INVENTION

Charged particle beam apparatuses have many functions, in a plurality ofindustrial fields, including, but not limited to, electron beam (wafer)inspection, critical dimensioning of semiconductor devices duringmanufacturing, defect review of semiconductor devices duringmanufacturing, exposure systems for lithography, detecting devices andtesting systems. Thus, there is a high demand for structuring, testingand inspecting specimens within the micrometer and nanometer scale.

Micrometer and nanometer scale process control, inspection orstructuring is often done with charged particle beams, e.g. electronbeams, which are generated and focused in charged particle beam devices,such as electron microscopes or electron beam pattern generators.Charged particle beams offer superior spatial resolution compared to,e.g. photon beams due to their short wavelengths.

Particularly for electron beam inspection (EBI) technology, throughputis of foremost interest. It is inter alia referred to, in particular, tosurface inspection at low landing energies <500 eV and low secondaryelectron (SE) extraction fields. Normally, for high current densityelectron probe generation, compound objective lenses are used(superimposed magnetic and electrostatic retarding field lenses). Inthose lenses, the electron energy inside the column is reduced to thefinal landing energy. Further, for the purpose of pre-charging a waferto a desirable surface potential, for example in order to increasedetection sensitivity of voltage contrast (VC) defects in the waferfabrication process, or to dis-charge/neutralize wafer charging effects,a flood gun can be used.

In view of the above, it is beneficial to provide an improved chargedparticle beam device and a method of operating thereof that overcome atleast some of the problems in the art.

SUMMARY OF THE INVENTION

In light of the above, an improved charged particle beam waferinspection system, an improved multi-beam wafer imaging system, and animproved method of operating a charged particle beam wafer imagingsystem according to the independent claims are provided. Furtheradvantages, features, aspects and details are evident from the dependentclaims, the description and the drawings.

According to one embodiment, a charged particle beam specimen inspectionsystem is provided. The system includes an emitter for emitting at leastone charged particle beam, a specimen support table configured forsupporting the specimen, an objective lens for focusing the at least onecharged particle beam, a charge control electrode provided between theobjective lens and the specimen support table, wherein the chargecontrol electrode has at least one aperture opening for the at least onecharged particle beam, and a flood gun configured to emit furthercharged particles for charging of the specimen, wherein the chargecontrol electrode has a flood gun aperture opening.

According to another embodiment, a multi-beam specimen inspection systemis provided. The multi-beam specimen inspection system includes acharged particle beam specimen inspection system. The charged particlebeam specimen inspection system includes an emitter for emitting atleast one charged particle beam, a specimen support table configured forsupporting the specimen, an objective lens for focusing the at least onecharged particle beam, a charge control electrode provided between theobjective lens and the specimen support table, wherein the chargecontrol electrode has at least one aperture opening for the at least onecharged particle beam, and a flood gun configured to emit furthercharged particles for charging of the specimen, wherein the chargecontrol electrode has a flood gun aperture opening. The multi-beamspecimen inspection system further includes at least one further emitterfor emitting at least one further charged particle beam, wherein thecharge control electrode has at least one further aperture opening forthe at least one further charged particle beam.

According to yet another embodiment, a method of operating a chargedparticle beam specimen imaging system is provided. The method includesbiasing a charge control electrode to a first potential, moving aspecimen support table for positioning a first portion of a specimenbelow a flood gun aperture opening in the charge control electrode,pre-charging the first portion of the specimen with charged particlesemitted from a flood gun, and moving the specimen support table forpositioning the first portion of the specimen below a first apertureopening in the charge control electrode, wherein the first apertureopening is aligned with an optical axis of an objective lens of ascanning charged particle beam unit.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments. The accompanying drawings relate to embodiments of theinvention and are described in the following:

FIG. 1 illustrates a schematic partial view of a scanning chargedparticle beam device with a flood gun according to embodiments describedherein;

FIG. 2 illustrates a schematic view of a scanning charged particle beamdevice with a flood gun according to embodiments described herein;

FIG. 3A illustrates a schematic view of a charge control electrode for acharged particle beam wafer inspection system according to embodimentsdescribed herein;

FIG. 3B illustrates a schematic view of a conductive mesh, which may beprovided to close an aperture opening in a charge control electrodeaccording to embodiments described herein;

FIG. 4 shows a schematic of a flood gun, which is provided in a chargedparticle beam inspection system according to embodiments describedherein;

FIGS. 5A and 5B illustrate schematic views of a calibration target,which can be utilized in a charged particle beam wafer inspection systemaccording to embodiments described herein;

FIG. 6 shows a schematic view of a multi-beam wafer imaging systemhaving a flood gun according to embodiments described herein; and

FIG. 7 shows a flow chart of a method of operating a charged particlebeam inspection system, according to embodiments described herein;

DETAILED DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to the various embodiments of theinvention, one or more examples of which are illustrated in the figures.Within the following description of the drawings, the same referencenumbers refer to same components. Generally, only the differences withrespect to individual embodiments are described. Each example isprovided by way of explanation of the invention and is not meant as alimitation of the invention. Further, features illustrated or describedas part of one embodiment can be used on or in conjunction with otherembodiments to yield yet a further embodiment. It is intended that thedescription includes such modifications and variations.

Without limiting the scope of protection of the present application, inthe following the charged particle beam device or components thereofwill exemplarily be referred to as an electron beam device including thedetection of secondary electrons and/or backscattered electrons, whichare also referred to as signal electrons. Embodiments can still beapplied for apparatuses, systems and methods, in which the chargedparticle beam may alternatively be an ion beam. Embodiments can still beapplied for apparatuses and components detecting corpuscles such assecondary and/or backscattered charged particles in the form ofelectrons or ions, photons, X-rays or other signals in order to obtain aspecimen image. Generally, when referring to corpuscles they are to beunderstood as a light signal in which the corpuscles are photons as wellas particles, in which the corpuscles are ions, atoms, electrons orother particles.

A “specimen” or “wafer” as referred to herein, includes, but is notlimited to, semiconductor wafers, semiconductor workpieces, and otherworkpieces such as memory disks, masks, substrates for flat paneldisplays and the like. According to some embodiments, a specimen can beselected from the group consisting of: a wafer, a mask, a substrate fora flat panel display, and a flat panel display. Embodiments of theinvention may be applied to any workpiece which is structured or onwhich material is deposited. A specimen or wafer includes a surface tobe imaged and/or structured or on which layers are deposited, an edge,and typically a bevel.

According to some embodiments, which can be combined with otherembodiments described herein, the apparatus and methods are configuredfor or are applied for electron beam inspection (EBI), criticaldimension measurement and defect review applications, where themicroscopes and methods according to embodiments described herein, canbe beneficially used in light of high throughput of these applications.According to some embodiments described herein, an E-beam inspection(EBI), critical dimension measurement (CD) tool, and/or defect review(DR) tool can be provided, wherein high resolution, large field of view,and high scanning speed can be achieved. According to embodimentsdescribed herein, a wafer imaging system or a wafer SEM inspection toolrefers to EBI tools, CD tools or DR tools, which are specific tools asunderstood by a person skilled in the art.

In the context of the here described embodiments, without limiting thescope of protection thereto, an intermediate beam acceleration systemintends to describe a charged particle beam apparatus with initial highacceleration of the charged particles which will be decelerated to alanding energy shortly before striking the specimen or wafer. The energyor velocity ratio v_(acc)/v_(landing) between the acceleration velocityv_(acc) at which the charged particles are guided through the column andthe landing velocity v_(landing) at which the charged particles strikethe specimen can be about at least 10 or higher, e.g. 20 or higher.Furthermore, the landing energy can be 2 keV or less, e.g. 1 keV orless, such as 500 eV or even 100 eV.

Embodiments described herein relate to systems being a single or multicolumn scanning electron microscope having a flood gun. The flood gun isprovided such that the objective lens and the flood gun shares a chargecontrol electrode and/or the flood gun is provided to be at leastpartially within the objective lens housing. The scanning electronmicroscope and the flood gun are combined together in one waferinspection apparatus. According to some embodiments, which can becombined with other embodiments described herein, the objective lens andthe flood gun share at least some electrostatic components and/or areprovided in a common magnetic environment. Accordingly, the throughputof a wafer inspection system can be further improved.

According to some embodiments, the flood gun is configured to generate ahigh emission current with a large spot size. The high emission currentand the large spot size enable scanning and charging of large surfacesto a desired potential in a short time. According to some embodiments,which can be combined with other embodiments described herein, theemission current of the flood gun can be up to 5 mA, for example, 50 μAto 500 μA, such as 100 μA to 300 μA. According to yet further additionalor alternative embodiments, the spot size in the plane of the specimen,e.g. a wafer, can be 7 mm or below, for example 3 mm to 6 mm, such asabout 5 mm. The beam energy of the flood gun can, according to someexamples, be 300 to 3000 eV.

Combining a flood gun in a scanning electron beam inspection systemaccording to embodiments described herein can be beneficial in light ofone or more of the following aspects. (1) The flood gun and the scanningelectron beam inspection system can share one or more of the opticalelements, for example the charge control electrode above the wafer.Accordingly, further power supplies and respective controllers may beshared. This can inter alia reduce the costs of ownership and/or thesystem complexity. (2) The provision, additionally or alternatively, ofa common magnetic environment by an objective lens housing can shieldmagnetic fields of nearby components, for example nearby SEM columns.(3) The common charge control electrode allows for charging the specimenor wafer surface utilizing the flood gun with the same chargingconditions as compared to the charging conditions of a column of theelectron beam inspection system. (4) The need for an alignment betweenthe charging system and the scanning system can be reduced. (5) Thestage movement between the charging system and the scanning system canbe reduced. Accordingly, the time for stage movement and/or navigationerrors can be reduced. (6) The specimen or wafer can be held at the samebias below the scanning system and the flood gun, wherein the cycle timebetween charging and scanning is reduced. Accordingly, it may even bepossible to pre-charge one portion on a wafer while scanning orinspecting another portion of the wafer. The above aspects allow forincreased throughput and/or reduced cost of ownership.

According to embodiments described herein, the objective lens for anelectron beam system, i.e. the last lens before the electron beamimpinges on the specimen or wafer, includes a magnetic-electrostaticlens. As shown in FIG. 1, the electrostatic lens component includes anupper electrode 162, which lies on a high potential and a lowerelectrode, e.g. charge control electrode 166, which lies on a potentialclose to the sample voltage and which decelerates the electrons forproviding the desired landing energy. These electrodes contribute tofocusing the beam as well as to slowing the beam down to the desired lowprimary beam voltage.

FIG. 1 shows a portion of a scanning electron microscope 100. Theobjective lens includes the magnetic lens assembly 60 having an upperpole piece 63, a lower pole piece 64 and a coil (not shown in FIG. 1).The objective lens further includes an electrostatic lens componenthaving a first electrode 162, i.e. upper electrode in the figures, and acharge control electrode 166, i.e. lower electrode in the figures.Further, a control electrode 170 for control of the signal electrons orthe extraction field acting on the signal electrons respectively isprovided at a position along the optical axis 2 from the position of thecharge control electrode 166 to the specimen support table 50 or thespecimen 52 respectively. In FIG. 1, the control electrode 170 isprovided within the charge control electrode 166. The control electrode170 can, for example, have essentially the same position along theoptical axis as the charge control electrode 166. The charge controlelectrode 166 can also be referred to as big proxi or large proxi andthe control electrode 170 can also be referred to as small proxi.According to some embodiments, the small proxi can be at the samedistance from the specimen as the large proxi. According to otherembodiments, the small proxi is closer to the specimen as the largeproxi.

According to the embodiments described herein, it is understood that thesmall proxi, i.e. the control electrode 170, has a small influence onthe properties of the electrostatic lens component, yet is sufficientlysmall enough to be considered an individual element, with thefunctionality to control the extraction of the SEs from the specimen orthe guidance of SEs released from the specimen.

The objective lens 60 focuses the electron beam 12, which travels in thecolumn along optical axis 2, on the specimen 52, i.e. in a specimenplane. The specimen 52 is supported on a specimen support table 50.According to some embodiments, which can be combined with otherembodiments described herein, scanning of an area of the specimen can beconducted by movement of the table in a first direction essentiallyperpendicular to the optical axis and by scanning lines in another,second direction essentially perpendicular to the optical axis andessentially perpendicular to the first direction.

A flood gun 152 is provided in the scanning electron microscope 100. Asshown in FIG. 1, the flood gun 152 shares the charge control electrode166 with the scanning inspection system of the scanning electronmicroscope. According to embodiments described herein, the chargecontrol electrode 166 has at least one aperture opening 162 and a floodgun aperture opening 154. The specimen support table can be moved to afirst position, in which the electron beam 12 impinges on the specimen52, for example a position as shown in FIG. 1. The specimen supporttable 50 can further be moved to a second position, in which chargedparticles emitted from the flood gun impinge on the specimen 52, forexample a wafer.

According to yet further embodiments, which can be combined with otherembodiments described herein, a charged particle beam wafer inspectionsystem, such as the scanning electron microscope 100 shown in FIG. 1,includes an objective lens housing 65. The objective lens housing 65surrounds the objective lens, and particularly the upper pole piece 63and the lower pole piece 64. For example, the objective lens housing 65is magnetically insulated from the pole pieces by an air gap or amagnetic insulator, i.e. a material with a relative permeability μ/μ₀=1,such as copper or the like. According to some embodiments, which can becombined with other embodiments described herein, the objective lenshousing 65 can include a material having a relative permeability μ/μ₀ of10000 or above, for example mu-metal or the like. The flood gun 152 andthe objective lens 60 share the same magnetic environment by having theobjective lens housing 65 surrounding at least a portion of the floodgun 152. The objective lens housing can shield the fields of nearby SEMcolumns or other devices for the objective lens 60 and the flood gun152.

According to some embodiments described herein, the objective lens, canbe electrostatic, magnetic or combined magnetic-electrostatic. Amagnetic lens or a magnetic lens assembly can be provided by a permanentmagnet, a coil, or a combination thereof. For example a the objectivelens can have a magnetic lens assembly including one or more polepieces. According to embodiments described herein, an objective lenshaving surrounds the objective lens and shields one or both of magneticfields and electrostatic fields. The objective lens housing surrounds atleast a portion of the flood gun. Accordingly, the flood gun can beplaced close to the objective lens.

Sharing at least one of the charge control electrode 162 and theobjective lens housing 65 allows for reduced costs and a small footprintof the inspection system. Further, the flood gun 152 can be provided ata distance from the scanning electron beam components such that thespecimen support table 50 can move the specimen 52 from the electronbeam to a position below the flood gun 152 and vice versa in a reducedtime.

Further embodiments can be described with respect to FIG. 2. FIG. 2shows a charged particle beam device, such as an SEM imaging apparatus,i.e. scanning electron microscope 100 having a flood gun 152. Theelectron beam column 20 provides a first chamber 21, a second chamber 22and a third chamber 23. The first chamber, which can also be referred toas a gun chamber, includes the electron source 30 having an emitter 31and suppressor.

According to embodiments described herein, the emitter 31 is connectedto a power supply for providing a voltage to the emitter. The emittercan be an emitter of one or more emitters of an emitter assembly. Forthe examples described herein, the potential provided to the emitter issuch that the electron beam is accelerated to an energy of 8 keV orabove. Accordingly, typically the emitter is biased to a potential of −8keV or higher negative voltages, e.g. in the case where the column andthe beam guiding tube, which also provides the first electrode 162 inFIG. 2, are on ground potential. However, higher beam energies insidethe column, e.g. 20 keV or higher, will be even more advantageous forthe electron optical performance (e.g. resolution or current density).As described above, having the emitter on a positive potential is atypical embodiment with the benefits that the column and the beamguiding tube can be at ground or at a moderate potential. Yet, withrespect to the focusing properties of the zoom lens according toembodiments described herein, the emitter could also be grounded and apower supply could be connected to the electrode 162 shown in FIG. 2.

An electron beam is generated by the electron beam source 30. The beamis aligned to the beam-shaping aperture 450, which is dimensioned toshape the beam, i.e. blocks a portion of the beam. Thereafter, the beampasses through the beam separator 380, which separates the primaryelectron beam and the signal electron beam, i.e. the signal electrons.The primary electron beam is focused on the specimen 52 or wafer by theobjective lens. The specimen is positioned on the specimen stage, i.e. aspecimen support table 50. On impingement of the electron beam, forexample, secondary or backscattered electrons are released from thespecimen 52, which can be detected by the detector 398. Even thoughbackscattered electrons and secondary electrons are typically detectedby the detector, some passages of this disclosure relate to secondaryelectrons only, i.e. as a comparison to primary electrons, and it isunderstood that backscattered electrons are also considered to be signalelectrons or similar to secondary electrons as understood herein, i.e.there are secondary products for signal generation of the image.

According to some embodiments, which can be combined with otherembodiments described herein, a condenser lens 420 and a beam shaping orbeam-limiting aperture 450 is provided. The two-stage deflection system440 is provided between the condenser lens and the beam-shaping aperture450 for alignment of the beam to the beam shaping aperture. According toembodiments described herein, which can be combined with otherembodiments described herein, the electrons are accelerated to thevoltage in the column by an extractor or by the anode. For example, theextractor can be provided by the first (upper) electrode of thecondenser lens 420 or by a further electrode (not shown). According toyet further embodiments, the condenser lens may also be a magneticcondenser lens for controlling the probe diameter.

Further, a scanning deflector assembly 370 is provided. For example, thescanning deflector assembly 370 can be a magnetic, but preferably anelectrostatic scanning deflector assembly, which is configured for highpixel rates. According to typical embodiments, which can be combinedwith other embodiments described herein, the scanning deflector assembly370 can be a single stage assembly as shown in FIG. 2. Alternatively,also a two-stage or even a three-stage deflector assembly can beprovided. Each stage of the deflector assembly can be provided at adifferent position along the optical axis 2.

Signal electrons, e.g. secondary and/or backscattered electrons, areextracted from the wafer or specimen e.g. by a control electrode and arefurther accelerated within the objective lens. The beam separator 380separates the primary electrons and the signal electrons. The beamseparator can be a Wien filter and/or can be at least one magneticdeflector, such that the signal electrons are deflected away from theoptical axis 2. The signal electrons are then guided by a beam bender392, e.g. a hemispherical beam bender, and a lens 394 to the detector398. Further elements like a filter 396 can be provided. According toyet further modifications, the detector can be a segmented detectorconfigured for detecting signal electrons depending on the startingangle at the specimen.

An objective lens housing 65 surrounds the objective lens 60. Further,at least a portion of the flood gun 152 can be surrounded by theobjective lens housing 65. According to some embodiments, the objectivelens and the flood gun can have a common objective lens housing. Thecharge control electrode 166 is provided between the wafer or specimen52 (or the wafer support table 50, respectively) and the commonobjective lens housing 65. This allows for controlling the chargingpotential of the specimen surface, e.g. the wafer surface, under theflood gun and the scanning electron microscope column. The voltagedifference between the wafer and the charge control electrode determinesthe resultant wafer potential. Flood gun electrons, i.e. the chargedparticles emitted from the flood gun, pass the charge control electrode166 through the flood gun aperture opening 154. According to someembodiments, which can be combined with other embodiments describedherein, the flood gun aperture opening can be covered with a mesh 254 orgrid. The mesh or grid can improve the uniformity of the electrostaticfield above the specimen. Accordingly, the uniformity of the chargingprofile can be improved with the mesh 254, e.g. a grid.

FIG. 2 shows a power supply 261 for the flood gun 152. The power supply261 is provided in an electrical cabinet 260. Further, a power supply262 is provided, wherein the charge control electrode 166 and the waferor the specimens support table 50, respectively, can be biased to thedesired potentials. Further, the controller 263 for controlling themovement of the specimen support table 50 can be provided. According toembodiments described herein, power supplies and individual controllerscan be controlled by a main controller 250, such as a main computerhaving at least a CPU and a memory.

According to yet further embodiments, which can be combined with otherembodiments described herein, a calibration target 240 can be providedon the specimen support table 50. Details of the calibration target 240are described with respect to FIGS. 5A and 5B. The calibration target240 is connected with the power supply 262 for biasing the specimen 52or the specimen support table 50, respectively.

The calibration target is configured for characterizing the beam ofelectrons emitted from the flood gun 152 and/or for measuring thecurrent emitted from the flood. The emission current of the flood guncan be up to 5 mA, for example, 50 μA to 500 μA, such as 100 μA to 300μA. The high emission current of the flood gun allows for a betterthroughput of the inspection system since pre-charging and/ordis-charging can be conducted in a much shorter time. According to yetfurther additional or alternative embodiments, the spot size in theplane of the specimen or wafer can be 7 mm or below, for example 3 mm to6 mm, such as about 5 mm. Accordingly, the current density is lower whenusing a flood gun, for example in the range of 1 to 10 μA/mm². Thisreduces the likelihood of having artifacts when inspecting a specimen,for example a wafer. Yet, the higher emission current allows forcharging some types of layers to the desired potential, which could notbe charged with the electron beam of the scanning electron beam column.Particularly, layers having a large capacitance may not be charged tothe desired potential with an electron beam of a scanning electronmicroscope.

Embodiments described herein can be utilized for or can includepre-charging a wafer to a desirable surface potential, for example inorder to increase detection sensitivity of voltage contrast (VC) defectsin the wafer fabrication process, and scanning an electron beam of ascanning electron beam microscope over the pre-charged surfacethereafter. According to some embodiments, the uniformity ofpre-charging over a scanned area can be 10 V peak-to-peak or below. Forexample, the specimen, such as a wafer, can be charged to 100 V±5V.

As shown in FIG. 2, the flood gun 152 uses some common interface withthe SEM column. The charge control electrode 166, which is configured tobe used during pre-charging, i.e. operation of the flood gun, and whichis configured to be used during inspection with a scanning electronbeam, is provided in the chamber 23. That is, the flood gun and the SEMcolumn are operated under the same vacuum condition, i.e. they share thesame pressure within the vacuum chamber.

The specimen support table 50 includes an X-Y-stage navigation system,which is configured to move the specimen, for example a wafer, under theSEM column and/or the flood gun. The wafer is biased to a voltagepotential, which determines the landing energy of the electrons on thewafer.

FIG. 3 shows a charge control electrode 166. The charge controlelectrode 166 has an opening 154. Charged particles, for exampleelectrons, which are emitted from the flood gun can pass through theopening 154 of the charge control electrode 166. The charge controlelectrode further includes openings 162. The example shown in FIG. 3Ashows five openings 162 for five electron beams of a scanning electronbeam system. The charge control electrode is also provided in thechamber 23, i.e. the vacuum chamber, as shown in FIG. 2, and is commonfor the SEM column and the flood gun. Accordingly, the charge control ofthe flood gun and the SEM column is controlled by the same high voltagecontroller.

A conductive mesh 254 is provided at the flood aperture opening orwithin the flood gun aperture opening. That is, the aperture opening iscovered with a thin metal mesh or a grid in order to generate a uniformand planar electrostatic field between the specimen, for example awafer, and the charge control electrode. The conductive mesh 254 can bebiased to the potential of the charge control electrode. By biasing theconductive mesh, a uniform and planar electrostatic field can beprovided. This improves the uniformity of the profile of thepre-charging.

As shown in FIG. 3B, the conductive mesh 254 includes a plurality ofwires 354. A first plurality of wires 354 extend in a first directionand a second plurality of wires 354 extend in a second direction, whichis different from the first direction. For example, the second directioncan be essentially perpendicular to the first direction. For example,the second direction can have an angle of 80° to 100° with respect tothe first direction. The first plurality of wires 354 and the secondplurality of wires 354 form the mesh 254.

According to some embodiments, which can be combined with otherembodiments described herein, the first direction of the first pluralityof wires and the second direction of the second plurality of wires isnot parallel to one of the specimen movement directions of the specimensupport table 50, which may for example move in an X-direction and aY-direction. Further, additionally or alternatively, the first directionof the first plurality of wires and the second direction of the secondplurality of wires are not parallel to one of the scanning directions ofthe charged particles emitted from the flood gun, which can be deflectedby a beam deflection system within the flood gun. The scanningdirections may also correspond to the X-director and the Y-direction ofthe specimen support table 50. Particularly, the first direction of thefirst plurality of wires and the second direction of the secondplurality of wires can have an angle of 30° to 60°, for example about45°, with respect to the X-direction or the Y-direction. Providing suchan angle can avoid an uncharged line on the surface of the specimen whenscanning the flood gun electrode over the specimen.

The conductive mesh 254 can have one or more protrusions 355. Theprotrusions 355 can be used to provide a fixed orientation of the firstdirection of the first plurality of wires and the second direction ofthe second plurality of wires with respect to the specimen movementdirection and/or scanning direction. The protrusions 355 may further beutilized for an electrical connection between the conductive mesh andthe charge control electrode. Yet further, the protrusions may serve foreasy replacement of the conductive mesh. A typical mesh may includewires having a thickness of 5 μm to 100 μm. A typical mesh may bemanufactured to have spaces between the wires of 80 μm to 200 μm. Theratio between the dimension of the wires and the dimension of this basisdetermines a blocking ratio, which may be 10% to 30%, for example about20%.

FIG. 4 shows the flood gun 152. The flood gun has a housing 402.Electrons are emitted from the emitter 404 and accelerated by the anode405. According to some embodiments, a beam blanker system 406 can beprovided. The beam blanker system can deflect the beam of electrons,such that the electrons are blocked by the beam blocker 407. Anelectrode 408 can be provided in order to focus the beam of chargedparticles. Accordingly, some embodiments may include a focusing option.The first beam scanning system 412 for deflecting the beam in a firstdirection, for example an X-direction, and the second beam scanningsystem 414 for deflecting the beam in a second direction, for example aY-direction, can be provided. The beam deflection system of the floodgun is configured to align the flood gun electrons to pass through thecenter of the aperture opening in the charge control electrode. Theaperture-opening diameter is limiting the flood gun beam size on thewafer plane to the desired size and shape. The diameter limitation canfurther serve to avoid charging in undesired regions.

FIG. 4 further shows the charge control electrode 166 having an apertureopening provided therein. The aperture opening in the charge controlelectrode 166 is closed by the conductive mesh 254. The electronsemitted from the flood gun 152 impinge on a specimen 52, for example awafer. According to embodiments described herein, the emission currentcan be controlled by controlling the source voltage. By controlling thesource voltage, a constant emission current can be provided. Thefocusing lens provided by the electrode 408, for example provided by thepotential of the electrode in combination with other electrodes andpotentials within the flood gun 152, allows for controlling the spotdiameter of the electrons emitted from the flood gun. Control of thespot diameter is beneficial in order to avoid flooding outside of thewafer surface during charging or the calibration target surface duringcalibration. Flooding outside of the desired region, for example oncables or the like may deteriorate the operation of the charged particlebeam inspection system due to the high beam currents emitted by theflood gun. The control of the flooding on the desired services canfurther be improved by the beam blanker 406 and/or the first beamscanning system and the second beam scanning system, which may deflectthe beam of electrons emitted from the flood gun.

FIG. 4 further shows the working distance 430, i.e. the distance of thelower portion of the housing 402 from the surface of the specimen. Thehousing 402 of the flood gun does not only provide a compartment for thecomponents provided therein but also defines the region in which thepotentials within the flood gun influence the electron beam of the floodgun. Accordingly, the working distance 430 is provided between the lowerportion of the housing 402 and the surface of the specimen 52. Accordingto some embodiments, which can be combined with other embodimentsdescribed herein, the working distance can be from 60 mm to 90 mm, forexample from 70 mm to 80 mm. The combination of beneficial choice of theworking distance with the conductive mesh allows for the desireduniformity of the electrostatic field between the wafer and the chargecontrol electrode. This allows the charging profile to be within adesired range of 10 V peak-to-peak or below.

FIG. 5A shows the calibration target 240. The calibration target 240 isfor example positioned on the specimen support table 50 as shown in FIG.2. The calibration target 240 can be utilized for characterizing thebeam of the flood gun 152 and/or for measuring the current emitted fromthe flood gun 152. In order to calibrate the flood gun, the specimensupport table 50 is moved such that the electrons from the flood gunimpinge on the calibration target 240. Accordingly, the calibrationtarget is located on the stage assembly. The calibration target allowsfor controlling the voltage potential of the specimen support table.According to embodiments described herein, the beam emitted from theflood gun can be characterized with the specimen support table, or thespecimen respectively, is biased to the operating voltage.

As shown in FIG. 5A, the calibration target 240 is connected to thepower supply 262 for biasing the wafer or the specimen support table,respectively. Variations in the power supply signal while scanning theflood gun spot over the calibration target can be analyzed. Thisanalysis allows characterization of the flood gun spot with the powersupply signals of the power supply 262. Contrary to the Faraday cup,which is typically grounded for measuring the current, the calibrationtarget according to some embodiments described herein operates under abiased condition.

The calibration target 240 has an aperture plate 540 with at least oneopening. The electron beam emitted from the flood gun can pass throughthe opening in the aperture plate 540. The electron beam impinges on acup 542. The cup 542 includes an electron absorbing material, e.g.conductive material. A current detection device 562 in the power supply262 provides the signal indicative of the current in the power supply.During flood gun calibration the current in the power supply 262 is zeroif the electron beam of the flood gun is switched off. If the electronbeam of the flood gun is switched on, the current can be detected in thepower supply. According to some embodiments, which can be combined withother embodiments described herein, the calibration target 240 is biasedduring the flood gun calibration. For example, the calibration target isbiased to the same potential as the wafer curing imaging and/orpre-charging of the wafer with the charged particle beam inspectionsystem. Since the target is biased to the potential of the wafer and/orthe specimen support table, there is a low risk of high voltage damages.

According to some embodiments, which can be combined with otherembodiments described herein, the aperture plate 540 can include atleast two openings. As shown in FIG. 5B the aperture plate 540 caninclude a small opening 551 and a large opening 552. The small opening551 has a diameter, which is smaller than the diameter of the electronbeam emitted from the flood gun. For example, the diameter of the smallopening 551 can be 1 mm or below. The large opening 552 has a diameterwhich is larger than the diameter of the electron beam emitted from theflood gun. For example, the diameter of the large opening 552 can be 5mm or above.

For the measurement of the total current, the calibration target 240 ispositioned, for example by moving the specimen support table, such thatthe electron beam emitted from the flood gun passes through the largeopening 552 of the aperture plate 540. The entire current is collectedin the cup 542 and the resulting current is measured in the voltagesupply 262. For characterizing the beam emitted from the flood gun, thecalibration target 240 is positioned, for example by moving the specimensupport table, such that the electron beam emitted from the flood gunpartly passes through the small opening 551 of the aperture plate 540.The position of the small opening 551 can be varied by scanning thespecimen support table, for example in X and Y directions. Since thesmall opening 551 is smaller than the beam diameter, only a portion ofthe electron beam emitted from the flood gun passes through the apertureplate and is collected by the cup 542. Measuring the current detected inthe voltage supply 262 as a function of the position of the specimensupport table, i.e. the position of the small opening 551, allows forgenerating a current profile of the electron beam. Accordingly, theelectron beam emitted from the flood gun can be characterized. Forexample, the shape of the electron beam can be measured with thecalibration target 240.

FIG. 6 illustrates yet further embodiments, wherein a retarding fieldscanning microscope, i.e. wafer imaging system 400 is provided as amulti-beam device. Typically, two or more beams can be provided in amulti-beam device. As an example, FIG. 5 shows five emitters 5 such thatfive electron beams are emitted in the gun chamber 520. This correspondsto the five aperture openings in the charge control electrode shown inFIG. 3A. The emitter tips are biased to an acceleration potentialV_(acc) by voltage supply 4, which provides a potential to the tips ascompared to ground 3. Electrodes 512, e.g. extractors or anodes can beprovided, e.g. in a cup-like shape. These electrodes are electricallyinsulated by insulators 532 with respect to each other and with respectto the gun chamber 520. According to some embodiments, which can becombined with other embodiments described herein, also two or more ofthe electrodes selected from the group consisting of extractor and anodecan be provided. Typically, these electrodes 512 are biased topotentials by voltage supplies (not shown) in order to control the twoor more electron beams.

The charged particle beams travel in a further chamber 530, in which aspecimen 52 is provided. The objective lens 560 focuses the beams on thespecimen or in a specimen plane, respectively. The objective lens canhave a magnetic lens assembly 60 with a common magnetic lens portion,i.e. a magnetic lens portion acting on two or more of the chargedparticle beams. For example, one common excitation coil is provided to apole piece unit or a pole piece assembly, wherein several openings forpassing of the two or more electron beams through the pole piece unitare provided. The one common excitation coil excites the pole pieceunit, such that, for example, one beam is focused per opening. Powersupply 9 can provide the current for the magnetic lens portion of theobjective lens.

As shown in FIG. 6, the objective lens 560 further includes anelectrostatic lens component 360. For example, an electrostatic lensportion 560 having one or more first electrodes and a second electrodeare provided. The second electrode can be a charge control electrodewith aperture openings for the scanned electron beams and an apertureopening for charged particles emitted from the flood gun. 152. As shownin FIG. 6, the first electrode can also be provided as a separatedelectrode for one or more of the electrostatic lens portions. That isthe first electrode can be separate and/or independent of a beam guidingtube in the column. This can also apply for the single beam columnsdescribed herein. Further, for each of the electrons beams, a controlelectrode can be provided.

Three power supplies 462, 466 and 470 are shown in FIG. 6. Some of thepower supplies have exemplarily five connection lines for respectiveelectrodes for each of the five electrostatic lens components. Forexample, power supply 462 can be connected to the respective firstelectrodes, power supply 466 can have a single connection to the commoncharge control electrode, and power supply 470 can be individuallyconnected to the respective control electrodes. The controller 460 isconnected to the voltage supplies 462, 466 and 470 for the electrodes ofthe electrostatic lens components and the control electrodes. Thevarious connection lines entering the column housing from some of thepower supplies (the rest of which is omitted for better overview)illustrate that each of the electrodes for the individual beams can becontrolled independently. However, it can be understood that one or moreof the electrodes of the electrostatic lens components and one or moreof the control electrodes can also be biased with a common power supply.Further, it is noted that particularly the power supply 462 can beomitted if the first electrode is grounded as explained above.

According to some embodiments, the objective lens can be providedaccording to any of the embodiments described herein. It has to beconsidered that particularly for EBI applications, but also for CD/DRapplications, as compared to common wafer imaging, throughput is acritical aspect to be considered. The operational modes described hereinare useful for high throughput. Also cold field emitters (CFE) andthermally assisted field emitters (TFEs) can be used to increase thethroughput. Accordingly, the combination of a flood gun according toembodiments described herein with a CFE, a thermally assisted fieldemitter, or a Schottky emitter is particularly useful. As a furtherimplementation, a combination with a multi-electron beam device as e.g.described with respect to FIG. 6 further provides a specificcombination, which can be considered beneficial for the throughput ofwafer inspection.

According to different embodiments, which can be combined with otherembodiments described herein, a multi-beam wafer inspection system caninclude two or more beams, wherein one beam each can be provided in twoor more columns, wherein two or more beams can be provided in onecolumn, or both, i.e. two or more columns can be provided, wherein eachof the two or more columns include two or more beams on the specimen,e.g. a wafer. If two or more columns are provided, they may share somecomponents, e.g. the charge control electrode. If two or more beams areprovided in one column they may be generated by a combination of amulti-opening aperture plate and a deflection system such that two ormore virtual sources are generated.

The embodiments described herein, may as well include additionalcomponents (not shown) such as condenser lenses, deflectors of theelectrostatic, magnetic or compound electrostatic-magnetic type, such asWien filters, stigmators of the electrostatic, magnetic or compoundelectrostatic-magnetic type, further lenses of the electrostatic,magnetic or compound electrostatic-magnetic type, and/or other opticalcomponents for influencing and/or correcting the beam of primary and/orsignal charged particles, such as deflectors or apertures. Indeed, forillustration purposes, some of those components are shown in the figuresdescribed herein. It is to be understood that one or more of suchcomponents can also be applied in embodiments of the invention.

According to some embodiments, a method of operating a charged particlewafer imaging system is provided. A flow chart of the method ofoperating a charged particle wafer imaging system is shown in FIG. 7.The method includes biasing a charge control electrode to a firstpotential as shown in box 702. A specimen support table is moved forpositioning a first portion of a wafer below a flood gun apertureopening in the charge control electrode (see reference numeral 704). Asindicated by box 706 the first portion of the wafer is pre-charged withcharged particles emitted from a flood gun. After pre-charging thespecimen support table is moved (see reference numeral 708) forpositioning the first portion of the wafer below a first apertureopening in the charge control electrode, wherein the first apertureopening is aligned with an optical axis of an objective lens of ascanning charged particle beam unit.

By pre-charging the first portion of the wafer with a flood gun beforeimaging the first portion with a scanning charged particle beam unit,the pre-charging can be conducted faster as compared to, for example,pre-charging with the scanning charged particle beam unit itself.Accordingly, throughput can be increased. Further, the first portion ofthe wafer is provided below the charge control electrode while beingmoved from the pre-charging position to the imaging position. This isbeneficial for improved charge control on the wafer to be inspected.

According to some embodiments, which can be combined with otherembodiments described herein, the specimen or wafer (or the specimensupport table, respectively) can also be biased to a specimen potential,for example a high potential, during imaging of the first portion of thewafer. Yet further, the specimen or wafer can be biased to the samespecimen potential during pre-charging of the first portion of thewafer. Accordingly, the potential difference, i.e. the voltage, betweenthe wafer and the charge control electrode is not varied when movingfrom the pre-charging position to the imaging position.

According to yet further embodiments, which can be combined with otherembodiments described herein, the specimen support table can be moved toa position such that the calibration target, which can be provided onthe specimen support table, is moved to a position below the flood gun.The charged particles, for example the electrons, emitted by the floodgun may then impinge on the calibration target. Impingement of electronsof the flood gun on the calibration target allows for measuring thecurrent of the flood gun and/or characterizing the electron beam emittedfrom the flood gun. According to some embodiments described herein, themeasurement of the current and/or the characterization of the electronbeam of the flood gun can be conducted while the calibration target isbiased to the potential, for example a high potential, such as thepotential provided to the wafer or specimen support table duringpre-charging and/or imaging of the wafer. For the measurement of thecurrent and/or the characterization of the electron beam of the floodgun, the current in the power supply for biasing the wafer or thespecimen support table can be measured upon impingement of electronsfrom the flood gun.

According to alternative embodiments, either the total current emittedfrom the flood gun can be measured with the calibration target or theprofile of the emission current can be measured for characterizing ofthe electron beam emitted from the flood gun. For measurement of thetotal current, the calibration target can include the large apertureopening in an aperture plate, which is larger than the beam diameter ofthe electron beam emitted from the flood gun. The entire beam of theflood gun can enter the calibration target for measurement of thecurrent. For measurement of an admission profile of the electron beamemitted from the flood gun, the calibration target can include a smallaperture opening in the aperture plate, which is smaller than the beamdiameter of the electron beam emitted from the flood gun. For example,the small diameter can be 2 mm or below, for example 1 mm or below, suchas about 0.5 mm. In light of the aperture being smaller than the beamdiameter, only a portion of the electron beam emitted from the flood gunpasses through the aperture opening. By scanning the small apertureopening relative to the electron beam emitted from the flood gun, byscanning the electron beam emitted from the flood gun relative to thesmall aperture opening, or by scanning both the electron beam emittedfrom the flood gun and the small aperture opening of the calibrationtarget, the current profile of the electron beam emitted from the floodgun can be measured.

According to yet further embodiments, which can be combined with otherembodiments described herein, the flood gun may also be provided fordis-charging of the portion of the wafer. For example, after positioningthe first portion of the wafer below the first aperture opening in thecharge control electrode and imaging the first portion of the wafer orat least area of the first portion of the wafer, charge may build up onthe area of the wafer upon imaging of the area of the wafer. Fordis-charging the area of the first portion of the wafer, the specimensupport table may be moved back for positioning the first portion of thewafer below the flood gun aperture opening. The area of the firstportion of the wafer can be dis-charged with the flood gun. The specimensupport table can be moved back to the imaging position. In the imagingposition, the imaging of the first portion of the wafer can becontinued.

Embodiments described herein refer to an imaging charged particle beamunit, wherein the focused charged particle beam is scanned over thespecimen, in combination with the flood gun, wherein the flood gun andthe imaging charged particle beam unit, for example electron beamcolumn, share the charge control electrode such as a proxi electrode.Further, the flood gun and the imaging charged particle beam unit canshare a power supply for biasing the charge control electrode. Accordingto yet further additional or alternative implementations, the flood gunand the imaging charged particle beam unit can share an objective lenshousing, such that a common magnetic environment is provided for theflood gun and the imaging charged particle beam unit. Based upon thesharing of the charge control electrode and/or the objective lenshousing, a beneficial distance between the flood gun and the imagingcharged particle beam unit, for example a scanning electron microscope,can be provided. This beneficial distance allows for sufficientseparation between the flood gun and the imaging charged particle beamunit. Yet, the flood gun and the imaging charged particle beam unit areclose enough to allow for increased throughput, for example based uponreduced movement time of the specimen support table. Beyond that,according to some embodiments, the distance may further allow forpre-charging or dis-charging of one portion of a wafer while anotherportion of the wafer is imaged with the imaging charged particle beamunit. Accordingly, throughput can be further improved.

According to yet further details of some embodiments, which can becombined with other embodiments described herein, the flood gun chargedparticle source can be heated to different temperatures during operationof the charged particle beam wafer inspection system. For example, theflood gun charged particle source, such as the flood gun electronsource, can be heated to an operation temperature for emitting chargedparticles, such as electrons. While the flood gun is not used foremitting charged particles, such as electrons, the flood gun chargedparticle source can be heated to a second temperature lower than theoperation temperature. The second temperature can be a temperaturesufficiently low enough such that no electrons are emitted from thecharged particle source of the flood gun. Lowering the temperatureduring and idle time of the flood gun enables increasing the lifetime ofthe flood gun charged particle source.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

The invention claimed is:
 1. A method of operating a charged particlebeam specimen imaging system, comprising: biasing a charge controlelectrode to a first potential; moving a specimen support table forpositioning a first portion of a specimen below a flood gun apertureopening in the charge control electrode; pre-charging the first portionof the specimen with charged particles emitted from a flood gun; andmoving the specimen support table for positioning the first portion ofthe specimen below a first aperture opening in the charge controlelectrode, wherein the first aperture opening is aligned with an opticalaxis of an objective lens of a scanning charged particle beam unit,wherein a path of a charge particle beam extends through the objectivelens and the first aperture opening, and a path of the charged particlesemitted from the flood gun extends through the flood gun apertureopening without passing through the objective lens.
 2. The methodaccording to claim 1, further comprising: scanning the charged particlebeam focused by the objective lens over an area of the first portion ofthe specimen.
 3. The method according to claim 2, wherein scanning thecharged particle beam is conducted while the charge control electrode isbiased to the first potential.
 4. The method according to claim 2,further comprising: biasing the charge control electrode to a secondpotential different from the first potential before scanning the chargedparticle beam.
 5. The method according to claim 1, further comprising:biasing the specimen to an operation voltage applied while scanning thecharged particle beam; moving the specimen support table for positioninga calibration target below the flood gun aperture opening in the chargecontrol electrode; and measuring a current of charged particles emittedfrom the flood gun while the calibration target is biased to theoperation voltage.
 6. The method according to claim 5, furthercomprising: moving the specimen support table while measuring thecurrent of the flood gun for characterizing a profile of the chargedparticles emitted from the flood gun.
 7. The method according to claim2, further comprising: pre-charging or dis-charging a second portion ofthe specimen while scanning the charged particle beam focused by theobjective lens over the area of the first portion of the specimen.